Thin, planar, and relatively large area light sources are needed in many applications. Backlights must often be provided for LCDs to make them readable in all environments. Thin backlights for LCDs are desired to preserve as much as possible the LCDs' traditional strengths of thin profile, low cost, and sunlight readability while permitting readability at numerous angles and in low light conditions. Lamps for use in the avionics environment, such as airplane cockpits, are preferably as lightweight, thin, and low power as possible.
Many demanding challenges exist for engineering a thin, planar source of uniform light. If incandescent lamps or LEDs are used as the light source, the optics for dispersing and diffusing light from the multiple point sources to the planar viewing surface must be provided to avoid local bright or dim spots. Additionally, provision must be made to dissipate the heat generated by the incandescent or LEDs, or alternatively, to utilize only high-temperature materials for LCDs.
Recent developments in large LED arrays have made them appear suitable for use in flat panel displays. However, arrayed LEDs still consume relatively high amounts of power and require careful attention to avoid the thermal effects from the LEDs. Furthermore, the problems of diffusing the light emitted by the LED arrays must still be overcome as well as the spectral limitations inherent in an LED.
The introduction, some thirty years ago of electroluminescent lamps, is a possible choice for a planar lamp. Unfortunately, electroluminescent lamps suffer from a short life at high frequencies and have low ultimate brightness at about one lumen per watt. Nevertheless, the electroluminescent lamp is sometimes selected as a solution to low light display outputs, despite its spectral limitations and intrinsic problems with life expectancy.
Another choice for generating light for a display is fluorescent technology. Fluorescent lamps have the advantage of being relatively efficient and capable of generating sufficiently bright light. Miniature fluorescent lights made for backlights are typically tubular structures having selected diameters and lengths. Backlighting schemes using tubular fluorescent lamps generally require a reflector and a diffuser to distribute the light. The additional weight and size of the light-directing components, when added to the bulb volume, result in a bulky package usually exceeding one inch in thickness. Furthermore, miniature fluorescent tubes are inherently very fragile and more costly to produce than the large-sized commercial counterparts. Despite the significant drawbacks of fluorescent tubes, they are often chosen to provide the backlighting required in today's LCD displays or aircraft cockpits.
Planar fluorescent lamps are well known in the art. Envelopes are formed by sealing molded glass pieces together along their edges. Some prior art planar lamps include labyrinthine discharge channels. See, for example, U.S. Pat. Nos. 3,508,103; 3,646,383; and 3,047,763. Because of the complex glass molding and stamped metal housings, the prior art fluorescent flat panels are difficult to manufacture and expensive. These lamps had nonuniform light intensity ouput across the lamp and were often too thick and too inefficient for portable computer screens using batteries.
One flat fluorescent lamp, as shown in U.S. Pat. No. 4,851,734 ('734), utilizes transparent electrodes on planar glass plates. Unfortunately, the narrow gap between the plates constricts the length of the positive column, resulting in low ultraviolet radiation and low illumination. Further, in the embodiment with the electrodes on the outside, the usable power is reduced because the glass must be sufficiently thick to withstand normal atmospheric implosion when the chamber is vacuum-evacuated. In the embodiment shown in FIG. 4 of the '734 patent, the electrodes are directly exposed to each other, with no insulating layer in-between, severely limiting their practical use. Further, the unprotected transparent thin-film electrodes sputter away very quickly from ionic bombardment within a fluorescent tube.
A flat fluorescent lamp designed for LCD backlighting is disclosed in U.S. Pat. No. 4,767,965. Two parallel glass plates are supported by a framepiece, including two cold cathode electrodes placed opposite each other. A plasma discharge at the optimum mercury vapor pressure ranges conducts current as an arc and not in a planar fashion. This results in a discharge which is nonuniform in the planar chamber and brightness variations as great as 60% across the face of the lamp. In addition, these parallel glass plates must be thick to avoid atmospheric implosion when a vacuum is drawn in the envelope.
Some problems of the prior art have been attempted to be overcome by using a combination of a hot cathode surrounded by a cold cathode in tubular fluorescent lamps as taught in U.S. Pat. Nos. 4,117,374 and 3,883,764. These lamps are designed for large currents and are opaque to visible light, thus exhibiting nonuniform dark areas at the lamp envelope ends.
A need remains for a planar lamp that is thin in cross-section and uniformly bright across the entire face thereof.